Nonablative state changeable data storage systems, for example, optical data storage systems, record information in a state changeable material that is switchable between at least two detectable states by the application of energy such as, for example, projected optical beam energy, electrical energy, or thermal energy, thereto.
State changeable data storage material is typically incorporated in a data storage device having a structure such that the data storage material is supported by a substrate and protected by encapsulants. In the case of optical data storage devices, the encapsulants include, for example, anti-ablation materials and layers, thermal insulation materials and layers, anti-reflection materials and layers, reflective layers, and chemical isolation layers. Moreover, various layers may perform more than one of these functions. For example, anti-reflection layers may also be anti-ablation layers and thermal insulating layers. The thicknesses of the layers, including the layer of state changeable data storage material, are engineered to minimize the energy necessary for effecting the state change and to optimize the high contrast ratio, high carrier to noise to ratio, and high stability of state changeable data storage materials.
The state changeable material is a material capable of being switched from one detectable state to another detectable state or states by the application of, for example, projected beam energy, electrical energy, or thermal energy thereto. The detectable states of state changeable materials may differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties, including indices of refraction and reflectivity, or combinations of one or more of the foregoing. The state of the state changeable material is detectable by properties such as, for example, the electrical conductivity, electrical resistivity, optical transmissivity, optical absorption, optical refraction, optical reflectivity, or combinations thereof. That is, the magnitude of the detectable property will vary in a predictable manner as the state changeable material changes state.
Formation of the data storage device includes deposition of the individual layers by, for example, evaporative deposition, chemical vapor deposition, and/or plasma deposition. As used herein plasma deposition includes sputtering, glow discharge, and plasma assisted chemical vapor deposition.
Tellurium based materials have been utilized as state changeable materials for data storage where the state change is a structural change evidenced by a change in a physical property such as reflectivity. This effect is described, for example, in J. Feinleib, J. deNeufville, S. C. Moss, and S. R. Ovshinsky, "Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors," Appl. Phys. Lett., Vol. 18(6), pages 254-257 (Mar. 15, 1971), and in U.S. Pat. No. 3,530,441 to S. R. Ovshinsky for Method and Apparatus For Storing And Retrieving Of Information.
Tellurium based state changeable materials, in general, are single or multi-phased systems and: (1) the ordering phenomena include a nucleation and growth process (including both or either homogeneous and heterogeneous nucleations) to convert a system of disordered materials to a system of ordered and disordered materials; and (2) the vitrification phenomena include melting and rapid quenching of the phase changeable material to transform a system of disordered and ordered materials to a system of largely disordered materials. The above phase changes and separations occur over relatively small distances.
In typical use, optical recording media have certain criterion which if improved will give better, faster or more reliable data storage. Some or these criterion are: 1) data storage density; 2) data erasability; 3) data storage location accuracy (jitter); 4) memory medium sensitivity; and 5) carrier to noise ratio. These criterion will now be discussed.
Data storage density refers to the amount of data that can be stored per unit area. With present PCE optical disk construction and usage, the data is stored as amorphous spots in a film of crystalline phase change material. These spots are typically about 0.6 microns wide and 1.2 microns long (along the length of the data track). These spot sizes and the data encoding algorithm used to arrange the spots along the track for optimal density and disk/drive interaction limit the upper end density of data that can be stored per unit area of the optical recording medium. Therefore, if the spot sizes and/or the spaces in between can be reduced, data density can be increased.
Data erasability refers to the ability to totally erase a spot on the memory medium layer. As mentioned above, the data is stored as amorphous spots on a crystalline film. When the spots are erased, the material in the previously amorphous spot is recrystallized. However, the material is not in the same crystalline state as the virgin crystalline material. The virgin crystalline material (which fills areas between and surrounding the data storage spots) is composed of relatively small, randomly oriented crystallites, while the recrystallized spots are composed of relatively larger crystallites which are generally oriented toward the center of the spot. Because of the differences in crystallite size and orientation, there is a difference in the reflectivity of the virgin crystalline material and the recrystallized spots. This reflectivity difference makes it more difficult to determine whether the spot has been erased or not, because it imparts residual read signal levels which appear as data signals. Therefore, if the reflectivity of the reflective material between and surrounding the data storage spots can be more closely matched to the reflectivity of the data storage material (in either its amorphous or recrystallized form) then the erasability (i.e. the detection of which state a particular spot is in) can be improved.
Data jitter relates to the minute fluctuations in the timing of the rise and fall of the electric signal generated by reflecting the read laser beam off the recorded media, as it moves under the laser beam, to an opto-electronic detector. In a typical optical recording medium, statistical variation causes the location of a data spot to fluctuate within a known range (i.e. the location of the spot jitters). Because of this statistical variation of the exact location of the data storage spots, reading the state of any recorded data spots becomes less precise. Therefore, an increased accuracy of data storage location (i.e. reduction in jitter) will make reading the disk more reliable.
Sensitivity relates to the amount of power required to transform the phase change medium from one state to the other. This in turn relates to the total volume of material required to be transformed. Therefore, smaller data recording spot sizes (i.e. smaller total volume required to be transformed) requires less laser power and increases the sensitivity.
Finally, carrier-to-noise ratio again is based on the difference in reflectivity between amorphous and crystalline material, and any other fluctuations in reflectivity. The different reflectivities of the virgin crystalline and recrystallized material increases, making determination of the stored data patern less reliable. By matching the surrounding reflectivity to that of one state of the data storage spots, the noise level contributed by the different crystalline structures is greatly reduced or even eliminated.
The data storage medium of the instant invention can improve any combination of these criterion.